In contemporary automotive systems it is often desirable to measure a pressure difference between two locations. For instance, it is desirable to measure a pressure difference across a sharp edge oriface in an EGR (exhaust gas reflow) system. Often, as in this case, the media can be very harsh. Because of this averse environment, isolation from the medium, here exhaust gas, is desirable to ensure that the sensor, typically semiconductor based, survives and functions properly over a long period of time.
Prior art approaches solved this challenge by using stainless steel diaphragms for sensing a pressure coupled by oil to a conventional semiconductor based pressure sensor. The stainless steel diaphragm provides the necessary isolation between the harsh media and the pressure sensor, and the oil provides the transfer of pressure to the pressure sensor. The oil medium used in this approach adds error to a pressure measurement because in the manufacturing process is difficult if not impractical to eliminate all air pockets. These air pockets add error to the pressure transfer between the stainless steel diaphragm sensing the media harsh pressure and the actual pressure sensor. Also, the oil pressure transfer performance is degraded with increasing temperature and time because of changes in oil viscosity and leakage of oil. Furthermore, using the oil filled approach is difficult to manufacture because the oil needs to be hermetically sealed between the stainless steel diaphragm and the pressure sensor.
What is needed is an improved media-isolated differential pressure sensor, that is more accurate, easier to manufacture, and has better field performance over time and temperature variations.